Method and apparatus for determining the total flow rate in a ventilation installation

ABSTRACT

A method and apparatus for determining the total flow rate of air flowing through a ventilation installation with a free suction fan (15) is described. The flow rate or a corresponding pressure is determined by measurement of the partial flow rate or a corresponding pressure at at least one measurement point in the vicinity of the inlet opening of the fan. A measurement tube (20) open at both ends is suitably used, and oriented with its longitudinal axis in the inflow direction, flow sensors being arranged inside the measurement tube. In this way, there is obtained good measurement accuracy within a large flow range.

FIELD OF THE INVENTION

The invention relates to a method and apparatus for determining thetotal flow rate in a ventilation installation.

BACKGROUND OF THE INVENTION

According to a primary aspect, the invention more particularly relatesto an installation provided with a free suction fan, the inlet(s) of thefan being at least partially defined peripherally by an annular inletwall portion that tapers in the inlet direction, and the flow ratebeing-measured on the suction side of the fan in the vicinity of theinlet opening(s) thereof. Such a method, as well as an apparatus forcarrying out the method is known from EP-A1-0419798 (GebhardtVentilatoren GmbH).

The performance of ventilation installations is dependent to anessential degree on the total flow rate. A given minimum flow rate isthus required in any ventilation installation for achieving the desiredindoor climate, particularly with respect to low pollution percentagesin the room air and desired room temperature by regulated supply of heator cooling with the air.

The greater requirements made on the indoor climate, the more importantit is to be able to measure, monitor and regulate the total and partialflows in the ventilation installation. If the total flow rate from acentral unit decreases by 10%, the partial flows to each room will alsodecrease by 10%. If monitoring of the total flow rate is enabled, thepartial flows in the entire ventilation installation can thus beindirectly monitored as well.

Several methods are known for flow measurement, particularly partialflows, but these methods either require an extra pressure drop withaccompanying energy increase, generation of noise and increasedoperational costs, or they require high flow rates for achievingsufficient measurement accuracy. Such high flow rates are not normallypresent in ventilation ducts, and in addition there are often obstacles,e.g. in the form of bends close to the measuring point. Therefore, it isgenerally not possible to achieve sufficient measurement accuracy withcertain simple flow meters, such as so-called Prandtl tubes (whichmeasure dynamic pressure, i.e. the difference between total pressure andstatic pressure) or temperature-responsive electrical components (e.g. aresistor, the resistance of which depends on the temperature and thusalso on the flow rate of the cooling air). For satisfactory measurementaccuracy within a large flow range there is further required that theflow meter is placed in a straight duct section with a distance of about5 duct diameters before and about 3 duct diameters after the measurementpoint.

In accordance with prior art, and as indicated above, the flow rate in aventilation installation may be determined on the basis of pressure-dropmeasurements which can be made at different places in the ventilationinstallation, e.g. in a duct system connected to a central unit, as isproposed in the published Swedish patent application SE-A-8704163-8 (ABBahco Ventilation). A pressure difference across a component in the ductsystem is measured here with the aid of pressure measurement outlets,the pressure drop across the component then being proportional to thesquare of the flow. The pressure measurement outlets are connected viahoses to a pressure-sensing means in a meter with a pointer for visualindication of the flow. This measurement method is however burdened withthe disadvantage that comparatively poor measurement accuracy isobtained, partly due to the comperatively low flow rate in the ducts andpartly due to practical installation difficulties.

Another method of flow determination is described in the Swedish patentspecification SE-C-455 442 (AB Bahco Ventilation). In this case a filterin a central unit is exchanged for two perforated plates serving asconstriction means, pressure sensors then being used to measure thedifferent pressure drops which occur with the filter in place on the onehand, and the constriction plates on the other. On the basis of theflows which have already been measured with the constriction plateslocated in a similar unit, the flow rate is interpolated or extrapolatedwhen the filter is in place, e.g. graphically with the aid of a diagram.

This method also gives comparatively poor measurement accuracy, and itcannot be used for continuous measurement during operation of theinstallation, at least not without considerable complications and workfrom personel.

Yet another known method is described in the published Swedish patentapplication SE-A-8701663-0 (Flakt AB), the pressure drop measurementbeing carried out on the suction side of a suction fan in a ventilationinstallation.

The fan is placed in an apparatus housing and on its pressure side isconnected to a duct system. A constriction means is arranged in theinlet portion of the housing on the suction side of the fan and has twopressure tappings connected to a differential pressure measurementdevice for determining the pressure drop and the flow rate.

The constriction means, e.g. in the form of adjustable baffles isadjustable between a completely open position and a maximum constrictionposition, i.e. the measurement position, which enables determination ofan empirical graph of the relationship between the measured pressuredifference in the measurement position and the corresponding flow rate.

This known apparatus thus requires a considerable constriction of thetotal flow rate during measurement while the apparatus is in operation,which results in increased need of energy, increased operational costsand disturbing noise.

Both in measurement of the total flow rate in or in connection with anair-conditioning unit (apparatus housing) and in measurement of thepartial flows in ducts it has therefore been necessary to compromiseenergy demand and measurement accuracy. No suitable method for readilydetermining the total flow rate in a ventilation installation has beenprovided so far. An attempt has been made to measure the pressure dropin a measurement chamber provided with constrictions and situated in theoutlet part of a fan (PCT/FI88/00149, publication No. W89/02581-Imatera)but even this method has turned out to be unsuitable, due to complicatedapparatus and comparatively high costs.

Attempts have also been made to measure the local flow rate in thevicinity of the inlet opening of a fan. See EP-A1-0419798 (GebhardtVentilation GmbH, mentioned in the opening paragraph) and JP-A-59 131116 (Nihon Furooseru K.K.) In both these cases, the measurement iscarried out by means of a tapping hole, made in the annular inlet wallportion, and an adjoining pipe or hose for sensing the static pressureat the surface of the inlet wall portion of the fan. However, in theregion closest to the surface of the inlet wall portion, the air flow issomewhat irregular and possibly subject to frictional disturbances,which depend on the exact geometrical configuration and the smoothnessof the surface and the flow rate. Therefore, the static pressuremeasured in such a way is generally not representative of the total flowrate, in particular when using damper control of the flow at a constantrotational speed of the fan. Moreover, of course, such a method can onlybe used upon modifying the inside structure of ordinary fans. There isalso risk of clogging of the tapping holes being freely exposed in theinlet.

SUMMARY OF THE INVENTION

Against this background, the present invention has the object ofproviding a method and apparatus enabling measurement of the total flowrate in a fan-driven ventilation installation without using specialconstriction means, which would affect the total flow rate to anessential degree, or using specially modified fans with an especiallydesigned inlet portion and internal measurement devices, while at thesame time securing reliable measurement results within a large flowrange, irrespective of what disturbances or constrictions that may bepresent in different parts of the ventilation installation. The methodand apparatus shall thus be generally useable in fan-driven ventilationinstallations and give good measurement results, even in existinginstallations by simple and inexpensive means.

Further objects are to enable reliable flow measurement with simplemeans to low cost, both in installation and in operation and service ofthe ventilation installation. In addition, it shall be possible to usethe method and apparatus without the requirement of exacting work bypersonel, e.g. for regular supervision of the installation.

These objects are achieved in accordance with the invention bydetermining the total flow rate on the basis of a representative partialflow rate being measured in a stable air flow region.

The invention is based on the understanding that the inflow conditionsare very stable for free suction fans, irrespective of whether it is aquestion of centrifugal fans (radial flow fans), axial flow fans orother types of fans, e.g. mixed flow fans. Free suction fans thus have aconical or otherwise inwardly tapering inlet wall portion, either formedas a part of a fan casing or in the form of a separate, annular element,so that the air flowing towards the fan wheel is guided smoothly into aninflow region leading directly to the fan wheel. This inflow region withparticularly stable air flow is restricted radially inside and at adistance from the above-mentioned annular inlet wall portion and axiallyoutwardly by an outer radial plane, which is located axially somewhatoutside the inlet wall portion, approximately at a distance of about 30%of the least diameter of the annular inlet wall portion. In this stableinflow region, the flow pattern of the air is maintained intactirrespective of large variations in the flow rate. At the same time, theflow rate is very high in the major portion of the inflow region, whichgives substantially better measurement conditions than in remainingparts of the ventilation installation connected to the fan.

The method and apparatus in accordance with the invention have beenfound in practical tests to give, inter alia, the following advantages:

negligible reduction of the total flow rate (less than 1% drop of thetotal pressure achieved by the fan);

simple installation, even for existing plants;

low installation and operational costs;

high versatility;

very high and stable measurement values (pressure values) withadvantageous, particularly square flow characteristic;

good measurement accuracy (measurement error less than ±5%) both withdamper control on the pressure or suction side of the fan (at constantrotational speed) as with rotational speed control of the fan;

no noise generation.

The practical tests have been carried out with different fan sizes(inlet from 110 mm to 350 mm) and with different types of fan wheels(forwardly as well as backwardly angled blades on centrifugal fans), allwith good results. Moreover, it appeared that various disturbances onthe inlet or outlet side of the fan had no effect on the measurementresults, e.g. normal belt drive arrangements or very heavy constrictionson the outlet side. A bend directly connected to the fan outlet has thusaffected the measurement accuracy by merely ±0.5%

The greatest measurement pressure values are obtained if the flowmeasurement means is placed axially in the vicinity of the annular inletwall portion of the fan, the flow characteristic then being practicallyquadratic. However, it is also possible to place the measurement meansaxially inside or slightly outside the region of the inlet wall portion.The characteristic is then changed to an exponential value m (accordingto the formula: Δp=k·q^(m)) up to about 3 or down to about 1.7, while atthe same time the measurement pressure values (or the pressuredifference Δp in pressure drop measurement, explained in more detailbelow) will be lower. The flow conditions are still stable, however, andconsequently it is quite possible to determine the total flow rate withgood accuracy after calibration.

The best results have been obtained, if a measurement tube open at bothends is placed in the immediate vicinity of the fan inlet opening, withthe longitudinal axis of the tube oriented substantially in the flowdirection and with the outlet end of the tube situated in the axiallycentral portion of the stable inflow region of the fan. Measurement isthen carried out by sensing of the partial flow axially through themeasurement tube, this partial flow rate being substantiallyproportional to the total flow rate.

In this manner, there are thus ensured good measurement conditions bysensing with a special measurement tube, in which the axial flow isprotected and alike for different flow rates. Also, the sensing means,e.g. the pressure tapping holes, are well protected inside the tube. Incheck measurements, it has been found that the partial flow rate throughthe measurement tube is substantially proportional to the total flowrate through the fan. The partial flow rate through the measurement tubeis accordingly representative and can be utilized as a measure of thetotal flow rate within a large flow range, i.e. the measurement will becorrect irrespective of what disturbances or constrictions that haveoccured in the ventilation installation.

Such a measurement tube suitably has small dimensions, and a smalldiameter in particular in relation to the area of the fan inletopenings, so that the total flow through the inlet opening is onlyaffected insignificantly by the measurement tube. The diameter of thetube is preferably only a fraction, preferably at most about 1/10 of thefan inlet opening diameter, and is suitably 10-20 mm.

In the above mentioned inflow region at the fan inlet, particularly inthe axially inward portion of this region, the air flow rate increasescompared with the flow rate axially outside this inflow region, thuscausing a greater static subpressure in the inflow region. Themeasurement tube is then suitably placed with its inlet end axiallyoutside the axially central portion of the inflow area, so that adriving pressure occurs between the ends of the tube. High flow ratesthrough the tube and extremely good measurement conditions are thusensured.

In principle, the partial flow through the measurement tube can bemeasured with the aid of a flow sensor, e.g. in the form of atransducer, such as a temperature-responsive electrical component, e.g.a resistor, the operational temperature of which is an unambiguousfunction of the cooling effect obtained in response to the flow rate.However, even in this case, it is also possible to sense some otherphysical parameter which is dependent on the flow.

A practically simple method of sensing the partial flow rate through themeasurement tube is to sense the air pressure with the aid of at leastone pressure sensing means placed adjacent to a constriction situatedinside the measurement tube and at a distance from each end. A pressuredifference can here be measured between two places, one immediatelyupstream and the other immediately downstream of the constriction (whichgives a particularly high pressure difference namely up to about 600 Paor even higher, and thereby very good measurement accuracy) or by thedifference in pressure immediately downstream of the constriction on theone hand and a reference point outside and upstream of the measurementtube on the other hand.

In the practical tests which have been made using measurement tubes ofthe kind described above, it has been found that the dimensions of thetube and the configuration of the constriction can vary somewhat withoutsubstantial alterations in the measurement values, i.e. themanufacturing tolerances are not critical and do not cause any greatcost.

The exact positioning of the measurement tube is not critical either,and mounting it can therefore be carried out without any special, costlycontrol arrangement.

The air flow rate is normally greater radially outside the central axisof the fan inlet opening than in the vicinity of its central axis. Themeasurement tube (or corresponding measurement means) is thereforesuitably placed at a radial distance from this central axis, preferablyat a radial distance amounting to about 20-90% of the least radius ofthe annular inlet wall portion.

In radial flow fans, the inflow conditions are different in theperipheral direction as a result of the normally spirally shapedexpanding configuration of the fan casing. The best measurement resultis obtained here if the measurement tube or corresponding measurementmeans is placed in the vicinity of the part of the fan inlet openingwhich is located substantially opposite to the fan outlet (the outletconnection of the fan casing) as seen in a peripheral direction.

Generally, the inventive concept also includes an apparatus fordetermining the total flow rate of air flowing through a ventilationinstallation, the apparatus being characterized by the combination of aflow measurement tube and a bracket for mounting the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, its characterizing features and advantages will now beexplained in more detail below with reference to the accompanyingdrawings, which illustrate a preferred embodiment example.

FIG. 1 schematically illustrates a unit in a ventilation installation,the unit including a housing in which are arranged fans, filters and aheat exchanger;

FIG. 2 is a perspective view of a fan included in the unit according toFIG. 1, the fan being provided with a measurement tube in associationwith its inlet opening;

FIG. 3 is an axial, partial section of the fan according to FIG. 2 withits associated measurement tube; and

FIG. 4 is an axial section through the measurement tube to a largerscale.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 there is schematically illustrated a unit 1 with a heatexchanger and duct connections 2,3,4,5 included in a ventilationinstallation. The unit 1 is situated in a housing 6 with intermediatewalls 7,8 so that four separate chambers 9,10,11,12 are formed. Thechamber 9 is connected to the duct connection 2 for outside air andaccomodates a filter 13 for filtering this air, which is caused to flowto the chamber 12 via a plate heat exchanger 14 centrally placed in thehousing 6. The flow is achieved with the aid of a supply air fan 15, andthe supply air is blown out by the duct connection 4 to an unillustratedduct system with supply air means (unillustrated) for supplying thesupply air in a building. From similarly unillustrated exhaust air meansthe air is sucked out from the building via a duct system to the ductconnection 3, from which this air flows through an exhaust air filter16, the chamber 10, the plate heat exchanger 14, chamber 11 and out viathe duct connection 5 by the action of an exhaust air fan 17 arranged inthe chamber 11.

The described ventilation installation is conventional, and onlyconstitutes an example. A problem present with most such equipment is tomaintain, upon installation and during operation, the desired flowrates, as discussed in the introduction.

In accordance with the present invention, the total flow rate, (i.e. thesupply air flow rate and/or the exhaust air flow rate) in theventilation installation is measured with the aid of a flow meter means,preferably in the form of a special measurement tube, which is placed inimmediate association with the inlet opening of a free suction fan.

In the embodiment example illustrated on the drawing (cf. FIG. 2 andFIG. 3 also), a measurement tube 20 is mounted on a bracket 21 adjacentthe inlet opening of the supply air fan 15. The fan 15 is a conventionalcentrifugal fan with double inlets and a fan wheel 22 provided withblades and formed as a double drum. Both parts 22a,22b (FIG. 3) of thedrum are connected by a common hub plate 22c in the centre of which isfixed the shaft 22d (FIG. 2) of the drive motor. A casing 23 surroundsthe fan wheel 22 and forms in a manner known per se a spirally shaped,peripherally expanding outlet duct, leading to a fan outlet 24. Thisoutlet is connected in an unillustrated manner to the duct connection 4in FIG. 1.

On either side of the fan wheel (seen in an axial direction) the casing23 forms an axially inwardly tapering, rounded wall portion 25 (only theright hand wall portion in FIG. 3 is visible in FIGS. 2 and 3), whichradially defines an inflow region I. In an axial direction the inflowregion I is defined by the lines denoted by A and D, the line D lyingaxially outside a line C (the radial plane through the outer end of thewall portion 25) at a distance a corresponding to 30% of the leastdiameter d (at the line B) of the opening.

The bracket 21 comprises an angled plate 21a fastened to the side wall26 of the fan casing 23, with a vertically dependent holder plate 21bfor carrying the measurement tube 20. If so desired, the plate 21aand/or the holder plate 21b can be adjustable for altering the positionof the measurement tube 20.

The measurement tube 20 is fitted with its longitudinal axis orientedparallel to the central axis L of the inlet opening and the fan wheel22, and has its outlet end 20a situated in the axially central portionI' (between the lines B and C) of the inflow region I relatively closeto the inlet wall portion 25, and substantially opposite to (in theperipheral direction) the fan outlet 24. The inlet end 20b of the tube20 is approximately conically expanding like a funnel. The diameter ofthe substantially circular-cylindrical measurement tube 20 is at mostabout 10% of the diameter d of the inlet opening, and the measurementtube thus takes up a very small part (about 1%) of the inlet openingarea. The length of the tube is preferably 5-10 greater than itsdiameter.

Inside the measurement tube 20 (see FIG. 4) there is a throttle plate 27with a central, circular through flow hole 28, the plate being insertedapproximately halfway between the ends 20a, 20b. On either side of thethrottle plate 27, immediately upstream and downstream of it, two smallholes 29,30 are bored in the cylindrical wall of the tube for pressuremeasurement. Pipe studs 31,32 for-connecting hoses 33,34, cf. FIG. 2also, are fixed round the respective holes 29,30.

When fan 15 is in operation, air is sucked in through the central inletopening. The flow has its greatest speed in the radially outward portionof the inflow area I. The measurement tube 20 is preferably arranged inthis outer portion, namely at a radial distance r from the central axisL corresponding to 50-90%, e.g. 70% of the radius (d/2) of the opening.However, when the measurement pressure is very high the measurementpoint can be placed closer to the central axis L, e.g. at a distance of20-50%. The flow direction of the inflowing air coincides substantiallywith the longitudinal axis of the measurement tube. A small portion ofthe total flow flows through the measurement tube 20, and the flow ratein this partial flow is at a maximum when the measurement tube is placedin accordance with the drawing figures.

With the aid of the pressure measurement tapping holes 29,30 and via thehoses 33,34, an unillustrated differential pressure measurement deviceof any suitable kind can sense the pressure drop across the throttleplate 27, and with this as a basis, both the partial flow rate and thetotal flow rate proportional to it can be determined. The pressure dropwill be relatively large since the sensed pressure upstream of thethrottle plate 27 in the pressure tapping hole 31 will be somewhatgreater than the static pressure at a greater distance upstream of thethrottle plate 27 inside the tube, e.g. at the point P1 in FIG. 4, andthe sensed pressure downstream of the throttle plate 27 in the pressuretapping hole 30 is somewhat lower than the static pressure at a greaterdistance downstream of the throttle plate 27 inside the tube, e.g. atthe point P2 in FIG. 4. This pressure drop increase at a throttle plateinside a tube is known per se, but is particularly advantageous toutilize in connection with the present invention. Accordingly, there canbe obtained very good measurement results within a large flow range.

The invention can be applied in many ways. By "free suction fan" thereis intended any fan, e.g. a centrifugal fan, an axial flow or amixed-flow fan, which has a free inlet on the suction side and which isthus not directly connected to a duct on the suction side. On the otherhand, the fan can naturally be accommodated in a chamber, which in turnis connected to a duct on the suction side, as is the case in FIG. 1.

By the expression "ventilation installation" is intended any apparatusor system which is connected to the fan in question. It may be aquestion of a very simple arrangement, e.g. merely a free-blowing fan,or a more complicated system of ducts on both the suction and pressuresides, with associated terminal devices and other equipment.

By "inlet opening" is intended any inlet opening through which air issucked into the fan. Centrifugal fans often have two opposing inletopenings, as in the embodiment example, and in such cases a flowmeasurement means can be placed adjacent one or both inlet openings. Itis just as well possible, of course, to arrange more than one flowmeasurement means at the same inlet opening. If two or more flowmeasurement means are used, a mean value of the measured magnitudes cansuitably be formed.

When a measurement tube is used, it does not need to be cylindrical, butmay be conical and/or have an expanded portion. In addition, it can besomewhat curved or comprise different mutually connected parts withsomewhat different orientation. What is essential is that themeasurement tube permits the air to flow through it in such a way thatthe partial flow rate will be substantially proportional to the totalflow rate.

Instead of mounting the measurement tube or corresponding flowmeasurements means on a bracket, radial holding arms arranged likespokes may be used. Alternatively, the flow measurement means in certaincases may be suspended at a shaft mounting for the fan wheel a, e.g.with belt driven fans.

The definition of the inflow region I (inside the line D in FIG. 3) hasbeen made for practical reasons. The radial plane D does not correspondto any actual, critical threshold effect, but serves as a practicaloutward limit for placing the flow measurement means, e.g. the outletend of the measurement tube, with retained measurement accuracy.

We claim:
 1. Apparatus for determining the total air flow rate in aventilation installation having a fan with a fan wheel, a fan inletopening and a fan outlet opening, said fan outlet opening beingconnected to a duct system and said fan inlet opening adapted to freelysuck in ambient air, and being defined by an annular inlet wall portion,which is centered around a central axis (L) and which tapers in the flowdirection so as to lead in inflowing air axially and radially towardssaid fan wheel, said apparatus comprising a stationary flow measurementtube mounted on a bracket in said fan inlet opening, said measurementtube having an open end for sensing a representative, local air flowrate at a measuring point, which is located inside said annular inletwall portion at a radial distance from said central axis (L), saidradial distance (r) being between 20% and 90% of the least radius (d/2)of said tapering inlet wall portion, and said measurement tube having adiameter which is no more than 20% of said least radius.
 2. Apparatus asclaimed in claim 1, wherein said measurement tube is constructed andarranged to sense an air pressure related to said representative, localair flow rate.
 3. Apparatus as claimed in claim 1, wherein said flowmeasurement tube is open at both ends and is placed with itslongitudinal axis oriented substantially in the flow direction and withits outlet end at said measuring point, sensing means being placedinside the flow measurement tube for sensing the partial flow rate ofthe air flowing axially through the flow measurement tube, said partialflow rate being substantially proportional to the total flow rate. 4.Apparatus as claimed in claim 3, wherein the inlet end of said flowmeasurement tube is situated axially outside said annular inlet wallportion.
 5. Apparatus as claimed in claim 3, further including aconstriction arranged inside the flow measurement tube between its ends,with at least one pressure sensing means being placed adjacent to theconstriction.
 6. Apparatus as claimed in claim 5, wherein two pressuresensing means are arranged respectively immediately upstream anddownstream of the constriction.
 7. Apparatus as claimed in claim 6,wherein said pressure sensing means situated adjacent to theconstriction comprise at least one pressure tapping hole which is incommunication with a differential pressure measurement device. 8.Apparatus as claimed in claim 5, wherein said constriction comprises anannular throttle plate having a central through hole.
 9. Apparatus asclaimed in claim 3, wherein the flow measurement tube is cylindrical andthe through hole is circular.
 10. Apparatus as claimed in claim 3,wherein the inlet end portion of the flow measurement tube is flaredoutwardly.
 11. Apparatus as claimed in claim 1, wherein said fan has acasing, and said bracket is fastened to a side wall of the fan casing.